The present invention relates to a digital copier and, more particularly, to a digital copier equipped with a binarizer for binarizing images of a document, which is scanned by a scanner, after deciding whether regions containing the images are character, symbol, figure and like image regions which need only simple binarization or photograph and like image regions which require halftone processing.
Generally, a digital copier uses a scanner which is implemented with a charge coupled device (CCD) image sensor. The scanner reads images of a document for each narrow region, i.e., on a pixel basis to produce analog image information. The analog image information is quantized with respect to mutiple levels and then converted to a digital signal. After being subjected to various kinds of processing, the digital signal is applied to a recorder of the copier to reproduce the document images.
In a copier of the type described, it is difficult to change the density level pixel by pixel while copying document images. To enhance reproducibility of images, it has been customary to binarize image information with respect to recording/non-recording, depending upon the characteristic of document images. However, since documents often carry photographs and others which need halftone processing (hereinafter referred to as halftone images) as well as characters, symbols, figures and others which need only simple binarization (hereinafter referred to as two-level images), it is also necessary to enhance reproducibility of halftone images. Various approaches such as a dither method, a density pattern method and a submatrix method have heretofore been proposed for rendering halftones. The problem with those approaches is that although photographs (halftone images) and others having slowly changing image densities can be favorably reproduced, characters (two-level images) and others having densities which assume any of two definite levels become illegible due to blurring which occur at their contours when reproduced and, in addition, unnoticeable contaminations in the background of a document sharply appear in copies. Such deteriorates the quality of reproduced images to a critical extent. Specifically, where a document carries both of two-level images and halftone images, simple processing all of them using two definite levels causes the halftone images to loose the gradations and, thereby, reproduces only inferior quality images, while halftone-processing all of them results in two-level images with blurred contours because two-level images are also subjected to halftone-processing.
To attain images of superior quality, therefore, it is desirable that two-level images be processed using simple two levels, while halftone images be subjected to halftone-processing as distinguished from the simple two-level or binary processing. Selection of simple binary processing and halftone processing has heretofore been implemented with a switch which is manipulatable depending upon the kind of a document, that is, presence/absence of halftone images which is decided by an operator, or logics for image recognition which uses a predetermined algorithm associated with characteristics of document images. Although this kind of implementation may be practiced relying on either software or hardware, software consumes a substantial period of time in recognizing images and, thereby, makes real-time image reproduction which is synchronous with the operation of a scanner impossible, while hardware, although shortening the processing time, results in a complicated construction.
Another approach to selectively execute the simple binary processing and the halftone processing known in the art is marks which are provided in document images in order to discriminate two-level images and halftone images, the two kinds of processing being switched from one to the other by sensing the marks during scanning. However, the mark scheme is undesirable from the operationability standpoint.
Another problem with the described type of copier is that where a line sensor or the like is used to read images pixel by pixel and if the density variation in a document is periodic, the period (pitch) often interferes with the pitches of arrangement of the sensor (sampling period) to develop moire in reproduced images. For example, where images are mesh-printed in a document, the periodic variation of the image density is apt to interfere with the sampling period of the sensor to develop moire. Assuming that the resolution of the sensor is 16 pixels per millimeter, moire often develops in a sensor output when the mesh-printing has a density close to the resolution of the sensor, i.e. 133 lines (about 10.5 pixels per millimeter) to 200 lines (about 16 pixels per millimeter). Although moire naturally occurs with other densities, too, its frequency is especially great when the density lies in the above-mentioned particular range, causing the signal to fluctuate over a significant range.
Mesh-printing itself is a kind of pseudo-halftone repesentation in which the pixel-by-pixel density variation occurs with two definite levels, i.e. ONEs (recording) and ZEROs (non-recording). In mesh-printing, the pitches and/or size of dots is variable to change the average density in multiple levels as a whole, thereby rendering halftone densities. Hence, disregarding the moire problem, a favorable reproduction of a mesh-printed document is attainable by processing a signal with respect to two levels. In practice, however, since moire cannot be eliminated when it comes to document images which are mesh-printed with the particular densities, the reproduction quality is critically lowered.
Meanwhile, where a sensor output representative of images is converted to a two-level or binary signal after halftone processing, reproduced images are free from or less affected by moire due to averaging of densities of a plurality of pixels, variation of a threshold level, and other operations included in the halftone processing. In this case, while the densities of images are provided in pseudo-halftone representation by dots, a mesh in a copy is not a direct reproduction of a mesh in a document but a mesh generated by halftone-processing which is particular to a copier. Therefore, where a document carries mesh-printed images or images which were reproduced by a copier using mesh-processing, it is preferable to select a copy mode which involves halftone processing despite that the pixels of the document images have two levels.
To prevent a particular part of a document from being reproduced in a copy as is often desired, customarily a white sheet is cut to produce a piece which is equal in size to the particular part of the document and, then, the white piece is bonded to the document to conceal the desired part. However, such a procedure is time- and labor-consuming. Moreover, the borders between the document and the white patch are often shadowed and appear in a copy as black lines, resulting in the need for an eraser for erasing the lines.
In a situation where characters, figures and others are printed in bold lines, one may desire to reproduce them as hollow images leaving their contours only, i.e. deleting their insides. In the image processing art, a technique is known which provides such a special image by preparing a blurred image and a sharp image and then providing exclusive-OR of the two different images by use of optical or electrical processing. Such processing, however, brings about various undesirable occurrences such as blurring, rounding of corners of images, and damaging small images.
Another known method available for producing hollow images as described above is binarizing an image signal using two different threshold levels and, then, providing exclusive-OR of the resulting binary image signals. This method, however, has the drawbacks that the line breadth is apt to become uneven, that the images become fat, and so on.